From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7C1CEC2BA83 for ; Wed, 12 Feb 2020 06:57:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 507AC2082F for ; Wed, 12 Feb 2020 06:57:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581490658; bh=19SiagyBycCgiJZF31FnbdIK3NQ4MFyvHwRPT//W9JE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=0wMjWT7qTDGnQjWMSxP512xjdDtaHDCbMEWYEJeOGunA4I1CIcLs3E3NJAypGqXuh Mo0cERwyXDDSKD4viRlBIxNcXIHl0Xht85RTYYEDP49fbLKd6qw4TehJmFowYxWkC7 O1r8a9dQZWWyYKU8yU3UY3inEI1ARXx0jxdbqBYw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728149AbgBLG5i (ORCPT ); Wed, 12 Feb 2020 01:57:38 -0500 Received: from mail.kernel.org ([198.145.29.99]:33294 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728148AbgBLG5h (ORCPT ); Wed, 12 Feb 2020 01:57:37 -0500 Received: from sol.localdomain (c-107-3-166-239.hsd1.ca.comcast.net [107.3.166.239]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E34E22073C; Wed, 12 Feb 2020 06:57:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581490656; bh=19SiagyBycCgiJZF31FnbdIK3NQ4MFyvHwRPT//W9JE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=h+dAaEROsZffCQMVY8G50cRWmHjM/wRDc/9JlSSWDsBr6/IAgHBKtnX222ZTx5Q2Q H77nJ9ax+MWcXCxYZ+7U/B4M3oavH5FuxwsoOg4BTYZI36prcxXHdxlvNgfXkOy/lv avaF1djrLaSPyjJTb1dEC5HGal0mzJPA5odqCW9M= Date: Tue, 11 Feb 2020 22:57:34 -0800 From: Eric Biggers To: Al Viro Cc: Daniel Rosenberg , Theodore Ts'o , linux-ext4@vger.kernel.org, Jaegeuk Kim , Chao Yu , linux-f2fs-devel@lists.sourceforge.net, linux-fscrypt@vger.kernel.org, Richard Weinberger , linux-mtd@lists.infradead.org, Andreas Dilger , Jonathan Corbet , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Gabriel Krisman Bertazi , kernel-team@android.com Subject: Re: [PATCH v7 2/8] fs: Add standard casefolding support Message-ID: <20200212065734.GA157327@sol.localdomain> References: <20200208013552.241832-1-drosen@google.com> <20200208013552.241832-3-drosen@google.com> <20200208021216.GE23230@ZenIV.linux.org.uk> <20200210234207.GJ23230@ZenIV.linux.org.uk> <20200212063440.GL870@sol.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200212063440.GL870@sol.localdomain> Sender: linux-fscrypt-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fscrypt@vger.kernel.org On Tue, Feb 11, 2020 at 10:34:40PM -0800, Eric Biggers wrote: > On Mon, Feb 10, 2020 at 11:42:07PM +0000, Al Viro wrote: > > On Mon, Feb 10, 2020 at 03:11:13PM -0800, Daniel Rosenberg wrote: > > > On Fri, Feb 7, 2020 at 6:12 PM Al Viro wrote: > > > > > > > > On Fri, Feb 07, 2020 at 05:35:46PM -0800, Daniel Rosenberg wrote: > > > > > > > > > > > > Again, is that safe in case when the contents of the string str points to > > > > keeps changing under you? > > > > > > I'm not sure what you mean. I thought it was safe to use the str and > > > len passed into d_compare. Even if it gets changed under RCU > > > conditions I thought there was some code to ensure that the name/len > > > pair passed in is consistent, and any other inconsistencies would get > > > caught by d_seq later. Are there unsafe code paths that can follow? > > > > If you ever fetch the same byte twice, you might see different values. > > You need a fairly careful use of READ_ONCE() or equivalents to make > > sure that you don't get screwed over by that. > > > > Sure, ->d_seq mismatch will throw the result out, but you need to make > > sure you won't oops/step on uninitialized memory/etc. in process. > > > > It's not impossible to get right, but it's not trivial and you need all > > code working with that much more careful than normal for string handling. > > It looks like this is a real problem, not just a "theoretical" data race. > For example, see: > > utf8ncursor(): > /* The first byte of s may not be an utf8 continuation. */ > if (len > 0 && (*s & 0xC0) == 0x80) > return -1; > > and then utf8byte(): > } else if ((*u8c->s & 0xC0) == 0x80) { > /* This is a continuation of the current character. */ > if (!u8c->p) > u8c->len--; > return (unsigned char)*u8c->s++; > > The first byte of the string is checked in two different functions, so it's very > likely to be loaded twice. In between, it could change from a non-continuation > byte to a continuation byte. That would cause the string length to be > decremented from 0 to UINT_MAX. Then utf8_strncasecmp() would run beyond the > bounds of the string until something happened to mismatch. > > That's just an example that I found right away; there are probably more. > > IMO, this needs to be fixed before anyone can actually use the ext4 and f2fs > casefolding stuff. > > I don't know the best solution. One option is to fix fs/unicode/ to handle > concurrently modified strings. Another could be to see what it would take to > serialize lookups and renames for casefolded directories... > Or (just throwing another idea out there) the dentry's name could be copied to a temporary buffer in ->d_compare(). The simplest version would be: u8 _name[NAME_MAX]; memcpy(_name, name, len); name = _name; Though, 255 bytes is a bit large for a stack buffer (so for long names it may need kmalloc with GFP_ATOMIC), and technically it would need a special version of memcpy() to be guaranteed safe from compiler optimizations (though I expect this would work in practice). Alternatively, take_dentry_name_snapshot() kind of does this already, except that it takes a dentry and not a (name, len) pair. - Eric